Industrial safety requires protection of operators, maintenance personnel, and bystanders from potential injuries from hazardous machinery or materials. In many cases the hazards can be reduced by automatically sounding an alarm or shutting off a process when dangerous circumstances are sensed, such as by detection of a person or object approaching a dangerous area. Industrial hazards include mechanical (e.g., crush, shear, impalement, entanglement), toxic (chemical, biological, radiation), heat and flame, cold, electrical, optical (laser, welding flash), etc. Varying combinations of hazards encountered in industrial processing can require numerous simultaneous safeguards, increasing capital expenses related to the process, and reducing reliability and flexibility thereof.
Machine tools can be designed with inherent safety features. Alternatively, hazards of machines or materials may be reduced by securing an enclosed machine or portions of the processing area during hazardous production cycles. Mechanical switches, photo-optical light-curtains and other proximity or motion sensors are well known safety and security components. These types of protection have the general disadvantage of being very limited in ability to detect more than a simple presence or absence (or motion) of an object or person. In addition, simple sensors are typically custom specified or designed for the particular machine, material, or area to be secured against a single type of hazard. Mechanical sensors, in particular, have the disadvantage of being activated by unidirectional touching, and they must often be specifically designed for that unique purpose. They cannot sense any other types of intrusion, nor sense objects approaching nearby, or objects arriving from an unpredicted direction. Even complicated combinations of motion and touch sensors can offer only limited and inflexible safety or security for circumstances in which one type of object or action in the area should be allowed, and another type should result in an alarm condition.
It is known to configure a light curtain (or "light barrier") by aligning a series of photo-transmitters and receivers in parallel to create a "curtain" of parallel light beams for safety/security monitoring. Any opaque object that blocks one of the beams will trigger the sensor, and thus sound an alarm or deploy other safety measures. However, since light beams travel in straight lines, the optical transmitter and receiver must be carefully aligned, and are typically found arranged with parallel beams. Light curtains are usually limited to the monitoring of planar protection areas. Although mirrors may be used to "bend" the beams around objects, this further complicates the design and calibration problems, and also reduces the operating range.
One major disadvantage of a light-curtain sensor is that there is a minimum resolution of objects that can even be detected, as determined by the inter-beam spacing. Any object smaller than the beam spacing could penetrate the "curtain" without being detected. Another disadvantage is that the light curtain, like most point-sensors, can only detect a binary condition (go/no-go) when an object actually interrupts one or more beams. Objects approaching dangerously close to the curtain remain undetected, and a fast-moving intruding object might not be detected until too late, thus forcing the designers to position the curtains further away from the danger areas in order to provide the necessary time-interval for safety measures. In addition, the safe operating range between the photo-transmitter and corresponding receiver can be severely limited in cases where chips, dust, or vapors cause dispersion and attenuation of the optical beam, or where vibrations and other machine movements can cause beam misalignment.
Furthermore, light curtains are susceptible to interference from ambient light, whether from an outside source, or reflected by a nearby object. This factor further limits the applications, making use difficult in locations such as outdoors, near welding operations, or near reflective materials. In such locations, the optical receivers may not properly sense a change in a light bean. Still further, light curtains are made from large numbers of discrete, sensitive, optical components that must be constantly monitored for proper operation to provide the requisite safety without false alarms. It is axiomatic that system reliability is reduced in proportion to the number of essential components and their corresponding failure rates. Microwave curtains are also available, in which focused microwave radiation is sent across an area to be protected, and changes in the energy or phasing at the distant receiver can trigger an alarm event. Microwave sensors have many of the same disadvantages of light curtains, including many false alarm conditions.
Ultrasonic sensor technologies are available, based upon emission and reception of sound energy at frequencies beyond human hearing range. Unlike photoelectric sensing, based upon optically sensing an object, ultrasonic sensing depends upon the hardness or density of an object, i.e., its ability to reflect sound. This makes ultrasonic sensors practical in some cases that are unsuitable for photoelectric sensors, however they share many common disadvantages with the photoelectric sensors. Most significantly, like many simple sensors, the disadvantages of ultrasonic sensors include that they produce only a binary result, i.e., whether or not an object has entered the safety zone. Similar problems exist for passive infrared sensors, which can only detect presence or absence of an object radiating heat.
Video surveillance and other measurement sensors are also known for use to automatically detect indications of malfunctions or intruders in secured areas. These types of known sensors are also limited to the simple detection of change in the video signal caused by the presence of an object, perhaps at some pre-defined location. These systems cannot detect the size or more importantly position of an object, since they are limited to sensing a twodimensional change in a scene. Another disadvantage of the video system is that it is limited to sensing motion or other change within the two-dimensional scanned scene, rather than other characteristics, such as the distance between objects. Furthermore, such systems cannot detect the number of intruding objects. They are unable to sense conditions under which a changed object creates an alarm condition, as opposed to an unchanged object, which will not create an alarm condition. Similarly, such systems disadvantageously can not provide an indication where two objects would be acceptable and one would create an alarm condition, and vice versa. Because of their simple construction, video systems are often used to periodically "sweep" an area, looking for changes. In this mode, the intruder can avoid detection by moving only when the camera is pointing away, and hiding behind other objects, creating the need for additional types of sensors to augment the video surveillance. Still further, video surveillance systems have no depth perception, and thus a small object near the camera could be perceived as equivalent to the image of a large object farther away. These and other disadvantages restrict the application of video surveillance systems, like the mechanical switch sensors, to simple, binary or "go/no-go" decisions about whether a new object has appeared.
More recently, proximity laser scanners (PLS) have been used to detect objects within a defined area near the PLS sensor. These systems are also known as Laser Measurement Systems (LMS). The PLS technology uses a scanning laser beam and measures the time-of-flight for reflected light to determine the position of objects within the viewing field. A relatively large zone, e.g., 50 meter radius over 180 degrees, can be scanned and computationally divided into smaller zones for early warnings and safety alarm or shutdown. However, like many of the other sensor technologies, the scanning laser systems typically cannot distinguish between different sizes or characteristics of objects detected, making them unsuitable for many safety or security applications. Significantly, the scanning laser systems typically incorporate moving parts, e.g., for changing the angle of a mirror used to direct the laser beam. Such moving parts experience wear, require precision alignment, are extremely fragile and are thus unreliable under challenging ambient conditions. Also, the PLS cannot discriminate between multiple objects and a single object in the same location. Nor can such systems detect the orientation and direction of the objects within the area being monitored. Thus, an object moving toward the target might raise the same alarm as an object in the same location moving away from the target, causing a false alarm in the PLS (or video surveillance, or other motion sensors). Also, the PLS cannot be used where a moving object is allowed in the area, i.e., the target object being protected is itself moving with respect to the sensor.